In high performance optical metrology and inspection systems, such as those used for semiconductor wafer inspection, the industry trend is to inspect a sample using multiple optical configurations in a serial fashion. The configuration options may include wavelength, illumination partial coherence (illumination sigma), and polarization state. In order to have high throughput, the optical configurations of the inspection system must be switched from one to the next in a very short time while imparting negligible levels of shock and vibration to the inspection system.
Although there are a number of types of polarizers, many types of polarizers are constrained to particular spectral bands and perform poorly with light outside the intended band. Thin-film polarizers, for example, are typically designed for narrow spectral bands, centered about a nominal wavelength. Wire grid polarizers work best primarily for handling light in the infrared region. Electro-optic and acousto-optic devices have limited spectral bandwidth and power ranges. The use of half-wave retarding plates in conjunction with static polarizers is not practical for broadband light and will not work for the full range of light that extends from deep ultraviolet (DUV, such as around 200 nm) to infrared (IR, such as around 800 nm). Still other polarizer types may work well in specific spectral regions and very poorly in others.
For performance over a broad range of wavelengths that include the DUV region, the selection is more limited. Polarizers that operate in this range and that meet the requirements for broad range operation are generally composite devices that use paired arrangements of birefringent crystals, coupled together along an interface and disposed at an appropriate position and angle in the light path for polarizing the incident light. Devices of this type, however, are typically large in volume and in mass. This makes it difficult to switch the polarization state quickly and without imparting some amount of shock and vibration to the inspection system.
Thus, although various types of polarization switching apparatus are used in narrow-band applications, these solutions are not suitable for broadband application, particularly where the spectral range spans the DUV wavelengths. There is, then, a need for a broadband polarization switching apparatus that is capable of handling light in the DUV region, but without requiring rotation of the polarizer itself as part of the switching operation.